MRI magnetic field generator

ABSTRACT

An MRI-use magnetic field generator structured such that there is no loss of magnetic field uniformity, temperature fluctuation is reduced and thermal efficiency enhanced, and the temperature of the permanent magnets can be controlled to high precision. With this invention, temperature control heaters are embedded in the base yokes of magnetic path formation members, and as a result of this structure, when the temperature control heaters are heated by a temperature regulator according to the temperature detected by a temperature sensor, the permanent magnets disposed in the vicinity of the base yokes are heated efficiently, so control follow-up is excellent. Furthermore, because the temperature control heaters are embedded inside the base yokes, and the heat generated by the heaters is conducted through the base yokes and reaches the permanent magnets directly, the heat is not diffused to the outside and lost, affording extremely efficient thermal control.

TECHNICAL FIELD

This invention relates to an improvement to a magnetic field generatorused in a medical-use magnetic resonance imaging device (hereinafterreferred to as an MRI device), and more particularly, this inventionrelates to an MRI-use magnetic field generator that controls thetemperature of permanent magnets that serve as the magnetic fieldgeneration source by measuring the temperature of the permanent magnetsand using a heating means or cooling means incorporated into base yokesor the like, so that any unevenness in the temperature distribution ofthe permanent magnets is efficiently reduced without any loss ofuniformity in the magnetic field generated within an imaging space.

BACKGROUND ART

An MRI device is designed so that all or part of a patient's body isinserted into the cavity of a magnetic field generator that forms apowerful magnetic field, and a body section image of the desired area isobtained, allowing a graphic representation to be made even of thetexture of the tissue of this area.

With the above-mentioned MRI-use magnetic field generator, the cavitymust be wide enough for all or part of the patient's body to beinserted, and a stable, powerful, and uniform magnetic field having aprecision of at least 1×10⁻⁴ at 0.02 to 2.0 T usually must be formedwithin the imaging space inside the cavity for a sharp body sectionimage to be obtained.

FIGS. 9A and 9B illustrate a known structure of a magnetic fieldgenerator used in an MRI device (Japanese Patent Publication H2-23010).Specifically, permanent magnets 30, which make use of R—Fe—B-basedmagnets as the magnetic field generation source, are fixed to theopposing sides of a pair of base yokes 35, pole pieces 31 are fixedopposite one another on the various magnetic pole sides, and a staticmagnetic field is generated within the cavity 33 between the pole pieces31. The illustrated magnetic circuit is achieved by connecting columnarsupport yokes 36 between the pair of flat base yokes 35. 37 in thefigure is a tilt field coil, and 38 is an imaging space formed in thecenter within the cavity 33.

The pole pieces 31 are usually made from a flat bulk material (a singlepiece) produced by planing down electromagnetic soft iron, pure iron, oranother such magnetic material. A structure in which an annularprotrusion 32 is provided around the periphery, or a protrusion isprovided in the center (not shown) (Japanese Utility Model PublicationH5-37446), or the like may be employed in order to enhance theuniformity of magnetic field distribution within the cavity 33.

Because of their relatively low maintenance costs, compact size, andother advantages, permanent magnets are increasingly being utilized asthe magnetic field generation source for forming a static magnetic fieldwithin the cavity 33. A drawback to these permanent magnets, however, isthat the field intensity tends to vary with changes in the temperaturedue to the magnetic characteristics inherent to the magnets themselves.

The stability of the intensity of the static magnetic field formed inthe cavity is important with an MRI device, and ways of keeping thefield intensity stable include covering the entire magnetic fieldgenerator, or just the required portions, with an adiabatic material sothat the permanent magnets are kept at a constant temperature, andproviding cooling or heating means on the inside of the base yokes orthe above-mentioned adiabatic material.

For instance, with a known structure in which a cooling means isprovided in order to reduce the effect that temperature changes have onthe static magnetic field generated by an MRI device, the temperature iscontrolled with a cooling apparatus in which electronic cooling elementsthat utilize the Peltier effect are disposed around the outer peripheryof the base yokes (Japanese Utility Model Publication H3-56005).Specifically, the above-mentioned cooling apparatus cools the entiremagnetic field generator to within a temperature range that is 10 to 50°C. lower than the ambient temperature, changes in the ambienttemperature are moderated by an adiabatic material that surrounds thedevice, and the temperature is fine-tuned to a specific range.

During diagnosis with an MRI device, the room temperature is usuallykept at about 22 to 25° C. so that a clothed patient can be examined incomfort. The structure described above requires that the MRI device bekept at all times at a temperature lower than room temperature, but thisis impractical because of inefficiency from the standpoint of energyconsumption, and because the structure for cooling the entire structuremakes the device larger and more expensive than with the structurediscussed below in which a heating means is provided.

A structure in which a heating means is provided makes it easier toobtain a compact and inexpensive device than the above-mentionedstructure in which a cooling apparatus is provided, and is said to bemore efficient in terms of energy consumption. Examples of suchstructures are known from Japanese Laid-Open Patent ApplicationsS63-43649 and S63-278310.

Specifically, it is common to use a structure in which any of variousheating means are used to control the entire magnetic field generator toa temperature that is about 5 to 10° C. lower than the room temperaturewhere the MRI device is installed.

The magnetic field generator shown in FIG. 10 is structured such thatflat base yokes 42 are disposed across from one another via columnarsupport yokes 43, permanent magnets 40 are fastened to the opposingsides thereof, and pole pieces 41 are provided to the magnetic polesides thereof. A planar heater 44 is disposed on the outer surface ofeach of the base yokes 42, a planar heater (not shown) is also disposedon the inner surfaces of the adiabatic materials 45, and these yokes areentirely covered with the adiabatic material 45.

With a structure such as this, electrical current is sent from a powersource (not shown), and the temperature of the magnetic circuit iscontrolled.

Japanese Laid-Open Patent Application S63-43649 proposes a structure inwhich planar heaters are disposed only on the inner surfaces of theabove-mentioned adiabatic materials 45. The problem with this structure,though, is that the temperature of the magnetic circuit is controlled byusing a fan to forcibly send air heated by the planar heater through anair passage formed between the flat base yoke 42 and the adiabaticmaterial 45, so not only is the device complicated, but because themagnetic circuit is heated via air, the thermal efficiency is also poor.

An object of the invention of Japanese Laid-Open Patent ApplicationS63-278310 is to solve the above problems, and as shown in FIG. 10, thethermal efficiency is improved somewhat by directly disposing the planarheaters 44 on the outer surfaces of the base yokes 42 on which thepermanent magnets 41 are disposed. However, because the heaters 44 aredisposed on the outer surfaces of the base yokes 42, that is, on thesides opposite from the cavity-facing sides of the permanent magnets 40,there is a pronounced tendency for the heat to be diffused from themagnetic circuit to the outside, so no improvement in thermal efficiencyis realized.

Furthermore, Japanese Laid-Open Patent Application H8-266506 (U.S. Pat.No. 5,652,517) discloses a structure that improves on the inventiondiscussed in Japanese Laid-Open Patent Application S63-278310. Thestructure of Japanese Laid-Open Patent Application H8-266506 ischaracterized in that a thermally conductive material is attached,either directly or via a gas, to the side faces of upper and lower baseyokes to which permanent magnets are attached.

The heater means in Japanese Laid-Open Patent Application H8-266506 isin the form of a sheet heater, and an AC sheet heater and a DC sheetheater are fixed one above the other to the side faces of the baseyokes. The fixing is accomplished by covering the AC sheet heater andthe DC sheet heater with a fixing bake plate from above and bolting theplate down.

Japanese Laid-Open Patent Application H8-266506 states that the abovestructure affords improvements in thermal efficiency, control follow-upproperties, and ease of work as compared to the structures disclosed inJapanese Laid-Open Patent Applications S63-43649 and S63-278310.

Still, because even the structure in Japanese Laid-Open PatentApplication H8-266506 makes use of planar heaters, there isfundamentally a great deal of thermal radiation to the sides oppositethe sides in contact with the yokes, so thermal efficiency is poor.Also, it is indicated that a temperature sensor is only disposed in thevicinity of the center on the top face of the upper base yoke, and thatthe temperature of all the planar heaters is controlled according to thetemperature detected by this lone temperature sensor. In other words,this structure involves controlling the temperature of the entiremagnetic circuit with a single control system, so there is a widetemperature variance, and uniformity of the magnetic field is also lost.

SUMMARY OF THE INVENTION

It is an object of this invention to solve the problems encountered inthe past related to the temperature control of permanent magnets, and itis a further object to provide an MRI-use magnetic field generatorstructured such that there is no loss of magnetic field uniformity,temperature fluctuation is reduced and thermal efficiency enhanced, andthe temperature of the permanent magnets can be controlled to highprecision.

As a result of various investigations aimed at finding a structure withwhich the temperature of the permanent magnets could be controlled tohigh precision, the inventors turned their attention to the fact thatbecause planar sheet heaters were employed in the past as heating means,for example, heat radiated out from sides other than those where themagnetic circuit was provided, resulting in poor thermal efficiency.With this in mind, the present invention was perfected upon discoveringthat the thermal efficiency can be improved and operating costs can bereduced by incorporating the temperature control means (mainly just theheating means, or the heating means and heat radiating (cooling) means)into the base yokes where the permanent magnets are provided, forexample, and that the follow-up properties of temperature control canalso be enhanced by disposing the above-mentioned heating means or othersuch temperature control means in the vicinity of the permanent magnet.

Specifically, this invention is an MRI-use magnetic field generator thatforms a magnetic circuit with magnetic path formation members andpermanent magnets serving as magnetic field generation sources, and thatgenerates a magnetic field within an imaging space, this MRI-usemagnetic field generator having temperature control means incorporatedinto the permanent magnets and/or the magnetic path formation members.

With the above-mentioned MRI-use magnetic field generator, the inventorsalso propose a structure in which temperature sensors are disposed inthe permanent magnets and/or the magnetic path formation members, astructure in which there is a temperature regulator which controls thetemperature of the temperature control means according to thetemperature detected by the temperature sensors, and a structure havingmeans for halting the temperature control means according to thetemperature of the permanent magnets and/or the magnetic path formationmembers.

Also proposed, as a particularly favorable structure, is a structure inwhich, in an MRI-use magnetic field generator in which a pair ofpermanent magnets are disposed facing each other via a cavity, there areat least two control systems that independently control the varioustemperatures of the pair of permanent magnets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front view illustrating the structure of the MRI-usemagnetic field generator of the present invention, and FIG. 1B is avertical cross section illustrating the main components in FIG. 1A;

FIG. 2 is an oblique view of the MRI-use magnetic field generator of thepresent invention;

FIG. 3 is a vertical cross section illustrating the main components ofanother example of the MRI-use magnetic field generator of the presentinvention;

FIG. 4 is a vertical cross section illustrating the main components ofanother example of the MRI-use magnetic field generator of the presentinvention;

FIG. 5 is a partial vertical cross section illustrating the maincomponents of the retaining means of the temperature control means usedin the MRI-use magnetic field generator of the present invention;

FIG. 6 is a partial vertical cross section illustrating the maincomponents of the retaining means of the temperature control means usedin the MRI-use magnetic field generator of the present invention;

FIG. 7A is a top view illustrating the structure of a pole piece of theMRI-use magnetic field generator of the present invention, and FIG. 7Bis a vertical cross section thereof;

FIG. 8 is a circuit diagram illustrating the temperature control of theMRI-use magnetic field generator of the present invention;

FIG. 9A is a partially cut-away front view illustrating the structure ofa conventional MRI-use magnetic field generator, and FIG. 9B is alateral cross section thereof; and

FIG. 10 is a partially cut-away oblique view illustrating the structureof another conventional MRI-use magnetic field generator.

BEST MODE FOR CARRYING OUT THE INVENTION

As long as its structure is such that a magnetic circuit is formed bymagnetic path formation members and permanent magnets that serve as themagnetic field generation sources, and a magnetic field is generated inan imaging space, the MRI-use magnetic field generator that is theobject of the present invention is not limited to the examples givenbelow, and can be applied to any structure.

For instance, the present invention can also be applied to a structurein which a pair of flat base yokes are linked by a plurality of columnarsupport yokes, a structure in which a pair of opposing, flat base yokesare supported at one end by a flat support yoke, a structure in whichpole pieces are disposed on the cavity-facing side of the permanentmagnets that serve as the magnetic field generation source, a structurein which no pole pieces are provided, and so forth.

The magnetic field strength, magnetic field uniformity and size of thecavity required by the magnetic path formation member dimensions of theflat base yokes should be suitably selected according to each of thevarious properties.

Ferrite magnets, rare earth cobalt-based magnets, or another knownmagnet material can be used as the permanent magnets that serve as themagnetic field generation sources. In particular, the device can be mademuch more compact by using an Fe—B—R-based permanent magnet in which Ris a light rare earth such as Nd and Pr, which are abundant resources,and in which boron and iron are the main components, exhibiting anextremely high energy volume of 30 MGOe or higher. A structure in whichthe above-mentioned known permanent magnets are disposed in combinationallows a more economical magnetic field generator to be provided withoutincreasing the size of the device.

Conventional materials such as electromagnetic soft iron or pure ironcan be used as the material for the yokes that serve as the magneticpath formation members. The use of base yokes allows the field intensityto be equalized, and also ensures that the entire magnetic circuit willhave good mechanical strength, facilitating the work of assembling thedevice.

The function of the support yokes is to mechanically support the baseyokes and ensure the required cavity dimensions, as well as to form amagnetic path for forming a magnetic field within the cavity.

The material of which the pole pieces are made is not limited to thematerials in the working examples. For instance, pure iron, or a softmagnetic powder that has been molded with an electrically resistantmaterial, or the like can be employed. The residual magnetism and eddycurrent generated at the pole pieces during application of a pulse fieldto the tilt field coils can be reduced by employing pole pieces madefrom a laminate of silicon steel sheets or from any of a variety of softferrites based on Mn—Zn, Ni—Zn, or the like, which have a low coerciveforce and high electrical resistance, or from a combination of thesematerials.

In particular, a laminate of silicon steel sheets is advantageous from acost standpoint because it is less expensive than a soft ferrite. Thereduction in eddy current and residual magnetism will be better and theattachment work will be easier if, as shown in FIGS. 7A and 7B, inproducing the pole piece 20 from the above-mentioned silicon steelsheets, a plurality of blocks 23 composed of laminates of silicon steelsheets are arranged on a magnetic material base 21, and these blocks arefurther laminated.

Optimizing the thickness of the above-mentioned entire pole piece or thethickness ratio of the magnetic material base 21 ensures that the polepiece will have good mechanical strength, equalizes the field intensityrequired of the pole piece, and prevents eddy current and residualmagnetism from occurring. It is also possible to employ a structure inwhich the magnetic material base 21 is not used by devising some meansfor fixing the blocks 23 composed of laminates of silicon steel sheets.

Furthermore, it is preferable to form an annular protrusion composed ofelectromagnetic soft iron, pure iron, or another such magnetic materialring around the periphery of the pole piece in order to enhance thefield uniformity within the cavity. In particular, the reduction in eddycurrent will be even better if one or more slits are provided to divideup the annular protrusion 22 in the circumferential direction as shownin FIGS. 7A and 7B.

The cross sectional shape of the annular protrusion is not limited tothe rectangular shape shown in the figure, and may instead beapproximately triangular, trapezoidal, or the like, with this shapebeing suitably selected as dictated by the required field intensity,field uniformity, and so forth. Disposing a protrusion 24 on the insideof the annular protrusion of the pole piece is also effective in termsof forming a uniform magnetic field.

The disposition of pole pieces in not essential in the presentinvention. Specifically, there are drawbacks to the use of pole pieces,such as a decrease in the field intensity within the cavity due tomagnetic flux leakage from the side faces of the pole pieces, a decreasein the tilt field rise characteristics due to the eddy current generatedwithin the pole pieces, and an increase in the weight of the overallmagnetic circuit, so a structure in which no pole pieces are disposed isalso effective in terms of avoiding these problems. The structure inwhich no pole pieces are disposed can be, for example, the structuredisclosed in Japanese Laid-Open Patent Application H3-209803, which waspreviously proposed by the inventors of the present invention.

Temperature control in the present invention is such that a temperatureregulator operates according to the temperature detected by atemperature sensor, and in the heating or heat radiation (cooling) bythe temperature control means, the temperature control means isincorporated into the permanent magnets themselves or into the polepieces or base yokes disposed in the vicinity of the permanent magnets,so less heat is lost through radiation to the outside, the permanentmagnets are heated and cooled extremely efficiently, and the follow-upproperties of the control are good. Furthermore, partial temperaturecontrol with a plurality of control systems is possible by disposing aplurality of temperature sensors, an advantage of which is less decreasein the symmetry of the magnetic field uniformity.

The temperature control means incorporated into the permanent magnets,base yokes, pole pieces, etc., in the present invention is not limitedto the structures illustrated in the working examples, and variety ofstructures can be employed so long as the temperature control means isdisposed in holes formed in the permanent magnets, base yokes, polepieces, etc., and these can be heated and cooled efficiently.

As described above, for a variety of reasons, a structure in which theentire magnetic field generator is controlled to a temperature about 5to 10° C. higher than the room temperature where the MRI device isinstalled is usually used at the present time. In the present inventionas well, it is preferable to employ a heating means as the temperaturecontrol means for reasons such as energy conservation, cost, and ease ofoperation.

From the standpoints of thermal efficiency, the service life of theheating means itself, and so forth, the heating means must be in closecontact with the members being heated, such as the base yokes.Alternatively, with disposition in holes, it is favorable to use aheat-resistant filler to achieve direct or indirect contact with themembers being heated.

A rod-shaped heating element is a particularly favorable device for theheating means because it can be easily inserted into holes formed in thepermanent magnets, base yokes, and pole pieces, and is also easy tohandle. Specifically, a rod-shaped heating element is a structure suchas a tubular heater comprising a heating element held within a metalpipe, with the space inside this pipe filled with an insulator such asMgO. Iron, copper, aluminum, stainless steel, or another such metal oran alloy material can be used for the above-mentioned metal pipe.

If these heating means move within the holes provided in theabove-mentioned base yokes or other heated members, or if they are takenout of the holes during use, there is the danger that the desiredtemperature control will be impossible, or that the heating meansthemselves could be damaged. It is therefore preferable to use aretaining means for the heating means as shown in FIGS. 5 and 6.

FIG. 5 illustrates a retaining means 53 in the form of a bolt that isthreaded into a hole formed in the base yoke 3. The rod-shaped heatingelement 10 that is inserted into the hole is held in place by thebolt-shaped retaining means 53 that strikes the end of a metal pipe 51that constitutes the rod-shaped heating element 10. In the figure, 52 isa lead wire going from the rod-shaped heating element 10 to the outside.

FIG. 6 illustrates a retaining means comprising a metal pipe 54 formedin an L-shape and disposed so that it strikes the end of the metal pipe51 that constitutes the rod-shaped heating element 10, and an attachmentbracket 55 that fixes the metal pipe 54 to the base yoke 3.

Various other structures besides the retaining means for a heating meansconsisting of a rod-shaped heating element shown in FIGS. 5 and 6 canalso be employed. For instance, threads may be formed around the outsideof the metal pipe 51 that constitutes the rod-shaped heating element 10,or a flange can be provided to the end of the metal pipe 51 for fixingthe base yoke 5, among other retaining means that can be employed.

It is also possible to use a cooling means as the temperature controlmeans in the present invention. For the sake of practicality and toprevent the device from being too large or costly, it is preferable toemploy a means with a simple structure such as a heat pipe.Specifically, cooling can be accomplished by disposing a heat pipe inholes provided to the permanent magnets, base yokes, or other magneticpath formation members by the same method as with the heating means andactively causing heat to radiate to the outside, or cooling can beeffected by introducing a coolant into the members via a heat pipe.

In order to control the temperature of the permanent magnets moreprecisely, it is also possible to use a rod-shaped heating element (theabove-mentioned heating means), a heat pipe (cooling means), or the liketogether.

The temperature sensors disposed for temperature control in the presentinvention can be temperature sensing resistors, thermistors, or thelike, with a known sensor being used as needed according to thestructure of the temperature control system. The temperature sensors maybe disposed at suitable locations as dictated by the structure of themagnetic circuit, such as the permanent magnets, base yokes, and polepieces.

Usually, the goal is achieved by disposing the temperature sensors onthe surface of the permanent magnets, base yokes, or pole pieces. Todetect the temperature at higher precision, it is favorable to formholes at specific locations of the various members mentioned above, anddispose the temperature sensors in these holes.

Particularly when the temperature sensors are disposed at the polepieces, it is preferable to dispose them in holes made at locations awayfrom the tilt field coils, such as around the outsides of the annularprotrusions, or in the centers of the pole pieces, since noise can begenerated by the magnetic field generated by the tilt field coils.

In addition to the circuit configurations given in the working examples,any known electrical control means can also be employed for thetemperature control of the permanent magnets by the above-mentionedtemperature control means and temperature sensor. A single controlsystem may be used, or two or more systems may be used as needed.

It is particularly favorable to use a plurality of control systems inorder to control the temperature of the entire magnetic circuit evenlyand without sacrificing the uniformity of the magnetic field. When themagnetic circuit needs to be raised from a relatively low temperaturestate to a specific temperature, a heating means with a large capacitycan be used concurrently in order to shorten the time it takes to raisethe temperature. In this case, it is preferable to use a temperatureregulator having two types of output: one for fast temperature elevationand one for fine tuning so as to maintain the temperature setting.

In the present invention, in order to utilize the above-mentionedtemperature control even more effectively, of the permanent magnets,base yokes, support yokes, and pole pieces that make up the magneticcircuit, the base yokes, which have a relatively large surface area andgreatly affect the temperature of the permanent magnet, shouldpreferably have disposed around their periphery an adiabatic materialfor isolating the heat from the air. Furthermore, besides the baseyokes, it is also favorable for the support yokes, permanent magnets,and pole pieces to be surrounded by an adiabatic material as needed.

Moreover, with the MRI-use magnetic field generator of the presentinvention, means can be provided for halting the operation of thetemperature control means should the temperature of the permanentmagnets rise markedly higher than the specified temperature due to amalfunction of the above-mentioned temperature sensor or temperatureregulator. For instance, to prevent the permanent magnets from beingheated to over 45° C., it is good to provide a thermostat for forciblyshutting off the current to the heater, and to prevent the constituentmembers of the magnetic circuit or the adiabatic material from beingburnt, it is good to provide a temperature fuse for forcibly shuttingoff current to the heater if the temperature exceeds 90° C., forexample.

Embodiment

The characteristics of the present invention will now be describedthrough reference to the examples illustrated in FIGS. 1A, 1B, and 2.

The magnetic field generator has magnetic path formation members, whichconstitute a magnetic circuit, disposed on a floor 1 via legs 2. Themagnetic path formation members comprise a pair of flat base yokes 3connected by four columnar support yokes 4. The magnetic fieldgeneration source consists of a pair of permanent magnets 5 featuringR—Fe—B-based magnets. These are attached to the opposing faces of thebase yokes 3, and pole pieces 6 are fixed to the respective pole piecefaces, forming a cavity 8 in which a static magnetic field is generatedbetween the pole pieces 6. An imaging space 9 is set up within thecavity 8 between the pole pieces 6, and a specific, uniform magneticfield is generated within this space. The pole pieces 6 that serve asmagnetic path formation members each have an annular protrusion 7 inthis structure, and are formed using the blocks of laminated siliconsteel sheets illustrated in FIGS. 7A and 7B.

Holes that are the same length as rod-shaped heating elements are madein the center of the four side faces and the top face or bottom face ofthe base yokes 3 (made of pure iron) in order to insert the rod-shapedheating elements. A plurality of rod-shaped heating elements (tubularheaters) 10 and 11 are inserted so as to be in close contact with holesformed in the base yokes 3, and are connected to a temperature regulatorvia leads and relays (not shown).

Here, these elements are connected to two-part temperature controlsystems 13 and 14 structured as shown in FIG. 8. With the temperaturecontrol systems 13 and 14, control signals from temperature regulators16 are sent to solid state relays 15, with this signal representing thedifference between the temperature setting and the temperature of thepermanent magnets as detected by the temperature sensor 12 disposedaround the outer periphery of the permanent magnets 5. Controlledcurrent passes through the solid state relays 15 to the rod-shapedheating elements 10 and 11, and suitable heating is performed accordingto the respective temperatures of the permanent magnets 5. As a result,a specified temperature is maintained, without any temperatureunevenness occurring in the magnetic circuit, and particularly theentire permanent magnets.

With a magnetic circuit as shown in FIG. 1A, in which permanent magnets5 are disposed facing each other on upper and lower base yokes 3, if thetemperature of the heaters disposed at the upper and lower base yokes 3is controlled just by the detection made by the temperature sensordisposed at one of the permanent magnets, then there will be a tendencyfor the temperature of the heater disposed in the base yoke where thepermanent magnet is disposed on the side with no temperature sensor tobe controlled somewhat lower than the optimal temperature.

To control the entire magnetic circuit to an even temperature, theseparate temperature control systems 14 and 14 shown in FIG. 8 must beprovided to the upper and lower base yokes 3. Specifically, theelectrical circuit is structured such that there are independent controlsystems 13 and 14, one for the temperature sensor 12 attached to thepermanent magnet 5 disposed at the upper base yoke 3 and the rod-shapedheating element 10 incorporated into the upper base yoke 3, and one forthe temperature sensor 12 attached to the permanent magnet 5 disposed atthe lower base yoke 3 and the rod-shaped heating element 10 incorporatedinto the lower base yoke 3.

A plurality of rod-shaped heating elements 10 and 11 are connected toeach of the control systems 13 and 14. This is to prevent local heatingof the magnetic circuit, and heat the overall circuit evenly. Also,although not shown in the figures, an adiabatic material for thermallyisolating the magnetic circuit from the surrounding air can be suitablydisposed.

The structure in FIG. 3 is such that a temperature control meansconsisting of a rod-shaped heating element 10 is incorporated not onlythe base yoke 3, but also into the pole piece 6. Specifically, anelectrical circuit is configured such that the rod-shaped heatingelement 10 incorporated into the pole piece 6 and the temperature sensor12 disposed on the pole piece 6 are integrated into a single controlsystem.

The structure in FIG. 4 is such that the permanent magnet 5 forms acavity for direct magnetic field generation. Specifically, a temperaturecontrol means consisting of a rod-shaped heating element 10 isincorporated into the base yoke 3 and the permanent magnet 5, and atemperature sensor 12 is provided to the cavity-facing side of thepermanent magnet 5. Here, an electrical circuit is configured such thatthe rod-shaped heating element 10 incorporated into the permanent magnet5 and the temperature sensor 12 disposed at the permanent magnet 5 areintegrated into a single control system.

As mentioned above, it is possible for the temperature control means tobe disposed in either the permanent magnets, the base yokes, or the polepieces. The temperature control means in the present invention isprovided in order to control the temperature of the permanent magnets,and a structure in which it is directly provided to the permanentmagnets is the most effective in terms of thermal efficiency.

Nevertheless, since subtle changes in the temperature of the permanentmagnets directly affect changes in the magnetic field, when thepermanent magnets are directly heated and cooled, it is preferable forthe detection of permanent magnet temperature by the temperature sensorsand feedback to the temperature control means to be carried outfrequently and in a short cycle.

Also, since heating more than necessary decreases the magnetic fieldintensity, controlling the temperature of the permanent magnets merelywith temperature control means disposed at the permanent magnets cannotbe considered a favorable structure. A preferable structure makes use oftemperature control means in the base yokes, pole pieces, and so on.

A structure in which the temperature control means is disposed at thebase yokes is not necessarily good in terms of thermal efficiencybecause the temperature of the permanent magnet is controlledindirectly. The base yokes, however, have a much larger volume than thepermanent magnets, and once they have been adjusted to a specifictemperature, they are not readily affected by changes in the ambienttemperature, so their temperature is more stable, and therefore thetemperature of the permanent magnet connected to the base yokes can beeasily kept constant. Also, because the base yokes are easier to machinethan the permanent magnets, the holes in which the rod-shaped heatingelements, heat pipes, etc., will be disposed can be formed at anylocation. It is therefore possible to keep the temperature uniform,without causing any temperature unevenness in the base yokes Themselves.

A structure in which the temperature control means is disposed at thepole pieces is not necessarily good in terms of thermal efficiency,either, because the temperature of the permanent magnet is controlledindirectly. Still, heating and cooling can be performed more efficientlythan with a structure involving disposition at the base yokes becausethe volume of the pole pieces is smaller and is about the same as thatof the permanent magnets. Furthermore, by controlling the temperature ofthe pole pieces, it is also possible to reduce the effect of temperaturechanges on the permanent magnets due to the generation of heat by thetilt field coils disposed in the vicinity of the pole pieces. Inparticular, by disposing a plurality of temperature control means atradial locations of the pole pieces, it is possible to keep thetemperature even for the entire pole pieces.

As mentioned above, it is also possible in the present invention for thetemperature control means to be incorporated in either the permanentmagnets, the base yokes, or the pole pieces. To keep the permanentmagnets at a constant temperature, it is preferable to select thecapacity, disposition location, disposition quantity, and so forth ofthe temperature control means after taking into account the variousvolumes, materials, and so on involved.

In the examples shown in FIGS. 1A, 1B, 3, and 4, the illustratedstructure makes use of a rod-shaped heating element as the temperaturecontrol means. In a similar structure, it is also possible to useconcurrently a cooling means featuring a heat pipe or the like.Specifically, a structure can be employed in which a cooling means isdisposed at the permanent magnets, or in which heating means and coolingmeans are both disposed at the base yokes.

Using the magnetic field generator of the present invention shown inFIGS. 1A and 1B, the target temperature of the upper and lower permanentmagnets 5 was set to 32° C. by the two-part temperature control systems13 and 14 shown in FIG. 8, whereupon the temperature differentialbetween the upper and lower magnets could be held to 0.1° C., and theenergy consumption was 600 W.

In contrast, with a magnetic field generator having a conventionalstructure in which a sheet heater was disposed on the outside of thebase yokes as shown in FIG. 10, because the control was effected by asingle temperature control system, the temperature differential betweenthe upper and lower magnets was 2 to 3° C., and the energy consumptionwas 1200 W.

Specifically, the structure of the present invention not only allows forhigh-precision temperature control, but also makes possible a majorreduction in energy consumption.

Also, using the magnetic field generator of the present invention shownin FIGS. 1A and 1B, the target temperature of the permanent magnets 5was set to 32° C. by a four-part temperature control system in whichtemperature control means corresponding to the structure of FIG. 3 werealso disposed at the pole pieces, whereupon it was confirmed that thetemperature differential between the upper and lower magnets could beheld to 0.1° C. even with respect to external temperature changes due toheat generated by the tilt field coils, etc.

INDUSTRIAL APPLICABILITY

The MRI-use magnetic field generator of the present invention ischaracterized by a structure in which temperature control means areembedded in the base yokes and 80 forth that make up the magnetic pathformation members. When the temperature control means executes heatingor cooling by means of a temperature regulator according to thetemperature detected by temperature sensors, the permanent magnetsdisposed in the vicinity of the base yokes or the like are heated orcooled efficiently, and follow-up with respect to control signals isgood.

Also, in the case of a temperature control means, such as a heater,embedded in the interior of the base yokes or the like, the heatgenerated by the heater is conducted through the base yokes, etc., andreaches the permanent magnets directly, so the heat is not diffused tothe outside and lost, allowing temperature control to be performedextremely efficiently.

Furthermore, partial temperature control is possible by disposing aplurality of temperature sensors at the permanent magnets. Anotheradvantage is that good symmetry can be achieved in the magnetic fielduniformity by controlling the temperature through control of thetemperature control means with a plurality of control systemsindependently provided to a plurality of permanent magnets.

What is claimed is:
 1. An MRI-use magnetic field generator whichcomprises: magnetic path-formation members that form a magnetic circuit,a plurality of permanent magnets which generate a magnetic field withinan imaging space located within said magnetic circuit, a heating meanslocated within at least one of said magnetic path-formation members andsaid plurality of permanent magnets, a temperature sensor for detectinga temperature of at least one of said magnetic path-formation membersand said plurality of permanent magnets, and a temperature regulator forregulating the temperature of said heating means based on a temperaturedetected by said temperature sensor.
 2. The MRI-use magnetic fieldgenerator according to claim 1, including a plurality of temperaturesensors disposed within the permanent magnets and/or the magnetic pathformation members, and wherein said temperature regulator controls thetemperature of said heating means according to temperatures detected bythe plurality of temperature sensors.
 3. The MRI-use magnetic fieldgenerator according to claim 1, including means for halting the workingof the temperature control means according to the temperature of thepermanent magnets and/or the magnetic path formation members.
 4. TheMRI-use magnetic field generator according to claim 1, wherein themagnetic path-formation members comprise a pair of base yokes that faceeach other to form a cavity having an imaging space, said permanentmagnets being disposed on their various cavity-facing sides, and supportyokes that connect and support these base yokes.
 5. The MRI-use magneticfield generator according to claim 4, including a plurality oftemperature sensors disposed in the permanent magnets and/or the baseyokes, and wherein said temperature regulator controls the temperatureof said heating means according to temperatures detected by thetemperature sensors.
 6. The MRI-use magnetic field generator accordingto claim 1, wherein the magnetic path-formation members comprise a pairof base yokes that face each other to form a cavity having an imagingspace, said permanent magnets are disposed on their variouscavity-facing sides, support yokes that connect and support these baseyokes, and including a pair of pole pieces disposed on the cavity-facingsides of the permanent magnets.
 7. The MRI-use magnetic field generatoraccording to claim 6, including a plurality of temperature sensorsdisposed at least in one of the permanent magnets and base yokes andpole pieces, and wherein said temperature regulator controls thetemperature of said heating means according to the temperature detectedby the temperature sensors.
 8. The MRI-use magnetic field generatoraccording to claim 5 or 7, wherein the temperature of the heating meansincorporated within the base yokes is controlled according to thetemperature detected by the temperature sensors disposed in thepermanent magnets.
 9. The MRI-use magnetic field generator according toclaim 5 or 7, wherein the temperature of the heating means incorporatedinto the permanent magnets is controlled according to the temperaturedetected by the temperature sensors disposed in the permanent magnets.10. The MRI-use magnetic field generator according to claim 5 or 7,including at least two control systems that independently control thevarious temperatures of the pair of permanent magnets.
 11. The MRI-usemagnetic field generator according to claim 5 or 7, including anadiabatic material covering at least a periphery of the base yokes. 12.The MRI-use magnetic field generator according to claim 7, wherein thetemperature of the heating means incorporated into the pole pieces iscontrolled according to the temperature detected by the temperaturesensors disposed in the permanent magnets or pole pieces.
 13. TheMRI-use magnetic field generator according to claim 1, wherein theheating means is a rod-shaped heating element.
 14. The MRI-use magneticfield generator according to claim 1, wherein the rod-shaped heatingelement includes retaining means.
 15. The MRI-use magnetic fieldgenerator according to claim 1, wherein the temperature control means isa heat pipe.